stream: add isReadable helper

Author: ronag

doc/api/stream.md

@@ -2238,6 +2238,19 @@ added: REPLACEME
 
 Returns whether the stream has encountered an error.
 
+### `stream.isReadable(stream)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `stream` {Readable|Duplex|ReadableStream}
+* Returns: {boolean}
+
+Returns whether the stream is readable.
+
 ### `stream.Readable.toWeb(streamReadable)`
 
 <!-- YAML

lib/internal/streams/utils.js

@@ -8,6 +8,7 @@ const {
 
 const kDestroyed = Symbol('kDestroyed');
 const kIsErrored = Symbol('kIsErrored');
+const kIsReadable = Symbol('kIsReadable');
 const kIsDisturbed = Symbol('kIsDisturbed');
 
 function isReadableNodeStream(obj, strict = false) {
@@ -116,6 +117,7 @@ function isReadableFinished(stream, strict) {
 }
 
 function isReadable(stream) {
+  if (stream[kIsReadable] != null) return stream[kIsReadable];
   const r = isReadableNodeStream(stream);
   if (r === null || typeof stream?.readable !== 'boolean') return null;
   if (isDestroyed(stream)) return false;
@@ -260,9 +262,11 @@ function isErrored(stream) {
 module.exports = {
   kDestroyed,
   isDisturbed,
-  isErrored,
   kIsDisturbed,
+  isErrored,
   kIsErrored,
+  isReadable,
+  kIsReadable,
   isClosed,
   isDestroyed,
   isDuplexNodeStream,

lib/internal/webstreams/readablestream.js

@@ -83,6 +83,7 @@ const {
 const {
   kIsDisturbed,
   kIsErrored,
+  kIsReadable,
 } = require('internal/streams/utils');
 
 const {
@@ -246,6 +247,10 @@ class ReadableStream {
     return this[kState].state === 'errored';
   }
 
+  get [kIsReadable]() {
+    return this[kState].state === 'readable';
+  }
+
   /**
    * @readonly
    * @type {boolean}

test/parallel/test-whatwg-readablestream.js

@@ -2,7 +2,7 @@
 'use strict';
 
 const common = require('../common');
-const { isDisturbed, isErrored } = require('stream');
+const { isDisturbed, isErrored, isReadable } = require('stream');
 const assert = require('assert');
 const {
   isPromise,
@@ -1573,7 +1573,6 @@ class Source {
   })().then(common.mustCall());
 }
 
-
 {
   const stream = new ReadableStream({
     pull: common.mustCall((controller) => {
@@ -1588,3 +1587,18 @@ class Source {
     isErrored(stream, true);
   })().then(common.mustCall());
 }
+
+{
+  const stream = new ReadableStream({
+    pull: common.mustCall((controller) => {
+      controller.error(new Error());
+    }),
+  });
+
+  const reader = stream.getReader();
+  (async () => {
+    isReadable(stream, true);
+    await reader.read().catch(common.mustCall());
+    isReadable(stream, false);
+  })().then(common.mustCall());
+}